Air flow control system in PFBC plants

ABSTRACT

Air flow is controlled in a PFBC plant comprising a combustor and a pressure vessel, a two-shaft gas turbine consisting of a low-pressure compressor LC and a low-pressure turbine LT interconnected by a shaft and a high-pressure compressor HC and a high-pressure turbine interconnected by a shaft. Air is sucked into LC, is passed on to HC and via an intercept valve into the pressure vessel and the combustor, whereafter the return flow is passed via the intercept valve to HT and further to LT, from where the return flow is released into the free environment. To control the air flow the set value (G set ) of the air flow control is calculated on the basis of the of the bed level and the actual value (G value ) is calculated from the available process parameters, and both are supplied to the regulator of the air flow control system, the output of which regulator controls the member for executing the control which is in the form of a guide vane ring with adjustable guide vanes and is provided at the inlet to LT.

TECHNICAL FIELD

The present invention relates to methods and devices for ensuring thatthe pressure vessel and the combustor of a PFBC plant are, at all times,pressurized with the aid of air flow means and that the proper quantityof air is supplied in each load case.

BACKGROUND OF THE INVENTION

PFBC plants have been described in a great number of publications andpatent specifications. One example of such a description is given in EP0124842 entitled "Power plant with a fluidized bed combustion chamber"and corresponding to U.S. Pat. No. 4,530,207. A so-called PFBC plantincludes a compressor unit and a turbine unit in which a low-pressurecompressor and a low-pressure turbine are interconnected via a firstshaft and a high-pressure compressor and a high-pressure turbine areinterconnected via a second shaft.

In a PFBC plant there is a relatively clear-cut relationship between thebed level of the combustor and the load. To be able to maintain thisrelationship and to obtain optimum operation, a large number of more orless connected systems are required. To cover the fuel consumed duringthe combustion, the proper quantity of fuel must be continuouslysupplied to the combustor. This is done with the aid of a fuel supplycontrol. The combustion of supplied mass for optimum use thereof and forkeeping the waste gases within permissible limits requires at all timesthe correct amount of air flow to the pressure vessel and to the bed ofthe PFBC plant. According to the above, the present invention comprisesa control system which satisfies these requirements.

Upon a change of load, in addition to both the fuel supply and the airflow being changed, also the bed level must be adapted to the new load.The bed level may be adjusted in several ways, among other things byfeeding bed material from the bed either out into a storage vessellocated outside the bed, or from the storage vessel into the bed. Thismethod is described, inter among other things in the above-mentioned EPspecification.

The system which supplies the combustor with compressed air comprises atwo-shaft gas turbine in which, as mentioned above, the low-pressurecompressor (LC) and the low-pressure turbine (LT) are interconnected bya fixed shaft and the high-pressure compressor (HC) and thehigh-pressure turbine (HT) are interconnected by a fixed shaft. Thehigh-pressure unit drives a generator via a star gear.

In addition to the system mentioned, PFBC plants comprise means forcontrol of bed temperature, feedwater and the like, which are all indifferent ways connected in dependence on the prevailing load and loadchanges. Most of these systems have limitations from the point of viewof process engineering. For the air flow control these comprise use thedifference pressure between the pressure vessel and the bed vessel aswell as the pressure ratio between the inlets and outlets ofcompressors, the minimum and maximum speed of the low-pressure shaft,the rate of change of several of the controlled quantities and the likewhich become active and influence each other.

The passage of the air through the compressors and up to the fluidizedbed as well as the passage of the corresponding waste gases through theturbines and out into the free environment are part of the prior art andwill therefore only be briefly described here. Air is admitted into thelow-pressure compressor and is passed on via an intermediate cooler tothe high-pressure compressor. From there the air is passed into thepressure vessel which thereby becomes pressurized with a certainoverpressure in relation to the combustor. The waste gases which aregenerated during the combustion are now passed from the combustor viacyclones to the high-pressure turbine and from there on to thelow-pressure turbine from where the gases are released, possibly aftercleaning and cooling, into the free environment.

The air and gas passage through the compressor and turbine units, asdescribed above, need not, of course, be associated with a fluidized bedplant. SE 8602003-9 corresponding to U.S. Pat. No. 4,893,466 entitled "Amethod for operation of a turbine unit" describes how such a turbineunit may be used together with other types of combustors for, forexample, operation of an electric generator connected to the commonshaft for the high-pressure compressor and the high-pressure turbine.From the design of this plant it can be deduced that the device forexecuting control of the power of the generator consists of a guide vanering, connected to the inlet of the low-pressure turbine, withadjustable guide vanes through which the gas flow from the high-pressureturbine and to the low-pressure turbine has to pass.

Otherwise, it is of interest to note that the above SE patent onlymentions devices for air supply under pressure to the combustor and doesnot disclose any form of the air flow control. As far as it is known, noindividual control of the air flow in known PFBC plants exists atpresent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the parts of a PFBC plant to which the invention isdirected, with the measuring points required for the different controls;

FIG. 2 shows a summary of the controls and limiting signals which areincluded in the air flow control system; and

FIG. 3 shows the configuration of the selection system which determineswhat control is to be switched on.

SUMMARY OF THE INVENTION

FIG. 1 shows the parts of a PFBC plant which constitute the mostimportant components of an air flow system, as largely described also inthe above-mentioned patent documents. Numeral 1 designates a combustorinside a pressure vessel 2. Air is sucked into the low-pressurecompressor 3 and passes through an intermediate cooler 4 on its way to ahigh-pressure compressor 5. From there the air is passed into thepressure vessel 2. The gas from the combustor is passed via cyclones(not shown) to a high-pressure turbine 6 and on its way out into thefree environment the gas passes through a low-pressure turbine 7. Thefigure also shows a storage vessel 8 with bed material used foradjustment of the bed level "h" in connection with load changes. Fuel isintended to be injected at the inlets 9. To the high-pressure shaftthere is connected an electric machine 10 which upon start-up mayoperate as a motor and which in normal operation may operate as agenerator. Output of power from tubes present in the bed is not shown inthe figure.

An air flow control according to the present invention comprises a newconcept for such a control since as a control executing member in theair flow control there is used a guide vane ring 11 with adjustableguide vanes on the input side of the low-pressure turbine. This meansthat the air flow is regulated by influencing the gas flow by means ofthe guide vane ring. The guide vane ring is the same as that whichaccording to SE 8602003-9 is used as executive member for power controlof the electric generator included therein. The operating member of theguide vane ring is shown at 12 and this member is influenced by thePID-connected air flow regulator 13. In the usual manner the air flowcontrol has a set value, G_(set), and an actual value, G_(actual). Theair flow is normally measured in kg/s.

The control concept described in FIG. 1 constitutes an exceedinglysimplified picture of the actual control system for the air flowcontrol. The reason for this are the above-mentioned limitations whichsuch a system must take into consideration. These will be described ingreater detail below under the description of embodiments of the presentinvention.

An important problem in an air flow control system is that the set valuemust automatically and at all times correspond to the current load, thatis, also correspond to the bed level. The relationship between air flowrequirement G_(set) and load or bed level may for a certain fuelmaterial be calculated as a function where also the temperature of thebed and the thermal value of the fuel and the like are included asparameters. Thus, for a certain given fuel the set value may be writtenas

    G.sub.set =f(h)

Whether the calculation of the necessary air quantity is based on theload or the bed level is largely irrelevant. It should be noted,however, that the load can be measured relatively accurately with moreor less conventional measuring methods. However, obtaining a measure ofthe current bed level is more difficult. One way is to measure thepressure difference between the pressure, P_(B), in the bottom of thecombustor and the pressure P_(O) in the so-called freeboard. The reasonis that there is a given relationship between the pressure differenceand the bed level "h". Pressure gauges used for this purpose are shownat 14 and 15 in FIG. 1.

Another and equally important requirement is to obtain a reliablemeasure of the actual value of the air flow. Among other things becausedirect-measuring air flow meters give rise to pressure drops across themeter itself, the current measured value is instead formed with the aidof known calculable relationships between measurable parameters. In thatconnection, a normalized speed N_(n) for the high-pressure compressor isfirst calculated according to the following relationship

    N.sub.n =N.sub.H √(416/T3)

where N_(H) is the physical speed, that is, the speed measured directlyon the high-pressure shaft, and where T₃ is the temperature of the airin kelvin before the high-pressure compressor. The respective measuringmeans are shown at 16 and 17 in FIG. 1. With the aid of producedmultitudes of curves for different normalized speeds, the normalizedcompressor air flow G_(n) versus the pressure ratio P₄ /P₃ across thehigh-pressure compressor can be determined. This pressure ratio iscalled π-value within the current technical field and will come into thepicture in a most obvious way in connection with some of the limitingcontrols which will be described below. Measuring means for thesepressures are shown at 18 and 19 in FIG. 1. The air flow through thehigh-pressure compressor can then be calculated according to thefollowing relationship

    G.sub.actual =G.sub.n P.sub.3 N.sub.n /3,63

where P₃ is the pressure of the air immediately before the high-pressurecompressor.

The speed N_(LC) of the low-pressure unit must not be below a givenminimum value or above a certain maximum value. The measured value forthis speed is included in the limiting control which continuouslyensures that the speed lies within permissible limits. The measuredvalue is obtained by a measured value transducer shown as 20 in FIG. 2.

The limiting controls also include the pressure difference between thepressure vessel and the combustor. A measured value of this differencepressure DP may be obtained with the pressure difference gauge 21.

FIG. 1 also shows a so-called intercept valve 22 through which the airflow to the pressure vessel and the gas flow from the combustor can beinfluenced. The task of the value will be explained in greater detailbelow under the description of the embodiments.

The different control limitations such as the π-control, the speedcontrol of the low-pressure compressor, the difference pressure controland the like are included together with the main control with thePID-connected air flow regulator in a selection system according to thepresent invention and the output signal of which corresponds to the mostpredominant of the controls and this signal is finally supplied to theoperating device 11 according to FIG. 1 for control of the adjustableguide vanes of the guide vane ring. The selection system works withanalog signals when it comes to deciding what control is to be switchedon. In addition, the selection system comprises logic signals whichrelate to the switching on and off of the air flow control, startup andrun-down of the plant as well as various faults which may occur. Theconfiguration of the selection system will be described in greaterdetail under the description of the embodiment.

It is, of course, important in a control concept of the type describedhere that the change between the different control systems when theserelieve each other takes place as smoothly as possible and without tooviolent "jumps" in the signal to the control executing member. Toaccomplish this according to the invention, the output signal from theselection system, that is, the control signal to the control executingmember, independently of what control is switched on, is fed back to allthe control systems as a follow-up set value which ensures that theoutputs of all the regulators have an output signal which only by agiven small margin differs from the output of that regulator which isswitched on.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a diagram of the measurement and control signals which areincluded in the air flow control and the limiting controls which areassociated therewith. Under the "Summary of the Invention" a descriptionis given of how both the set value and the actual value for the air flowcontrol is produced. The production of these quantities are thereforeshown only symbolically in FIG. 2 in the form of the set valuecalculator 23 and the actual value calculator 24. Input signal to theset value transducer is, in the example shown, the bed level "h" whichis assumed to have been obtained in some known way, for example via thepressure difference measurement between P_(O) and P_(B) as previouslydescribed.

The determination of the actual value for the air flow is performed, asmentioned above, with the aid of measured values for pressure upstreamand downstream of the high-pressure compressor, the temperaturedownstream of the high-pressure compressor and the speed of thehigh-pressure unit, which values are supplied to the actual valuecalculator which, with access to stored characteristics for normalizedair flow of the high-pressure compressor for different π-values fornormalized speeds thereof, may supply an actual value for air flow.

FIG. 2 also shows the PID-connected air flow regulator 13 according toFIG. 1. The output of the regulator, that is, the control signal to theoperating member of the guide vane if this control is switched on, issupplied to the selection system 25 which will be described in detailbelow. As mentioned, the selection system determines which controlsystem is to be switched on and the output signal from the selectionsystem is supplied to the operating device 11 of the guide vaneaccording to FIG. 1. To avoid an intermittent transition between thedifferent control systems when the selection system determines thatanother control means is to become activated, each control will besupplied with a follow-up set value Fset, as described above and shownin FIG. 2, which continuously ensures that each control system receivesa control output signal which differs, by a small deviation only, fromthe control signal which is activated.

Under normal operation the compressors must operate within a certainoperating range. If the working point approaches the limits of theworking range, problems with so-called surging and choking may arise.These phenomena may lead to considerable damage on the compressor andmust therefore be avoided. If the working point of the compressor shouldarrive outside the working range and below the curve corresponding tothe choking limit, choking occurs, and if the working point shouldarrive outside the working range and above the curve corresponding tothe surging limit, surging occurs. The air flow control is thereforesupplemented by a limiting control referred to as π_(HC) -control,symbolically shown at 26 in FIG. 2. To obtain a certain margin to theactual limit curves, this control is based on a π-surging limit curvewhich lies below the actual limit curve for surging and a π-chokinglimit curve which lies above the actual limit curve for choking. As isotherwise clear from FIG. 2, this control has as input signal the π_(HC)-value and the normalized speed N_(N) of the high-pressure compressor,which both values are obtained in connection with the production of theactual value for the air flow control as well as the pressure after thehigh-pressure compressor. The π_(HC) -control comprises two regulators,one of which supplies a control signal "Min π" and the other a controlsignal "Max π" which, when the operating range starts to approach therespective limit regions, exert an influence on the air flow control insuch a way that these signals, via the selection system, are determiningfor the position of the guide vane ring. In the same way as for the PIDregulator, the π_(HC) -regulators are supplied with the follow-up setvalue F_(set) to avoid disturbing jerks in the control when any of thesecontrols is switched on.

A brief description as to when the surging and choking problems ariseand how the Max-π and Min-π values are obtained for the different airflows will now be given. On each one of the curves for normalized speedsin the multitude of curves which provides the relationship between π andthe compressor air flow there is a point where surging and choking,respectively, are initiated. If the points for surging on all the curvesin the multitude of curves are linked together, a coherent curve isobtained which is called the surging curve, and if in similar manner thepoints for choking are linked together, a coherent choking curve isobtained. The region between these two curves defines the permissibleworking range of the compressor. To provide margins to these limitcurves, π-limit curves according to the above have been decided. TheMax-π regulator then sees to it that a set value for the Max-π controlis generated which has such a value that the working point of thecompressor by a certain margin does not come too close to the π-surginglimit curve, that is, lies below the π-surging limit curve. Suitably, amaximum curve is assumed which is determined by a curve equal to 0.9times the π-surging limit curve. In a corresponding manner, the Min-πregulator provides a set value for Min-π control when an operatingposition approaching the region for choking is about to be obtained. Forreasons of process technique a margin in relation to the π-choking limitcurve has been selected which is dependent on the pressure after thehigh-pressure compressor, that is, P₄, and for that reason also ameasured value for this pressure is supplied to the π-regulator. Whenthe pressure after the high-pressure compressor is greater than acertain pressure, for example 3.5 bar, a Min-π value is used which isdetermined by the π-choking limit curve and when the pressure is lower,a curve which is a few per cent lower than the π-choking limit curve isused. However, this value, of course, also permits a satisfactory marginin relation to the actual choking limit curve.

As already mentioned, the low-pressure unit is not allowed to operate atspeeds below a certain minimum speed or above a certain maximum speed.If there are tendencies in that direction, the limitation is to takeover the control of the guide vane ring via the selection system. Themeasured value for the speed of the low-pressure unit, as it can beobtained from the transducer 20 in FIG. 1, is therefore supplied to theN_(LC) control 27 in FIG. 2. From there a signal "Minspeed LC" isobtained if the actual speed drops to the lowest permissible speed and asignal "Maxspeed LC" is obtained if the speed is increased to thehighest permissible. One of these signals will determine the mode ofrunning of the air flow system via the selection system if this signalis the predominant one of all control signals that is, having thehighest control priority. As is clear from the figure, also this controlis supplied with the follow-up set value F_(set).

As previously described, an electric machine 10 according to FIG. 1 isconnected to the shaft of the high-pressure unit. During normaloperation this machine operates as a generator and delivers electricalpower to the power network. The same machine may also advantageously beused as a motor for start-up of the high-pressure unit and then drawspower from the network. In order not to overload the shaft between thehigh-pressure compressor and the turbine, however, the power from thenetwork to the machine operating as a motor must be limited. A measuredvalue of this power is therefore supplied to a maximum power controldevice 28 according to FIG. 2. This control device operates in exactlythe same way as the DP control. This means that the output signalfollows the output signal of the selection system because of thefollow-up set value F_(set) for as long as the input signal, that is MW,lies below a maximally allowed value set in advance. When the suppliedpower amounts to the maximally allowed value, a control signal "Max MW"is obtained which, in the same way as for the other control systems, issupplied to the selection system.

The air flow which is supplied to the pressure vessel tends to give thisvessel an overpressure in relation to the combustor. The differencepressure must be limited to a maximum value and the measured value DP ofthe difference pressure DP, obtained with the aid of the transducer 21according to FIG. 1, is therefore supplied to the difference pressurecontrol device 29 according to FIG. 2. This control device operates inthe same way as the other control systems in that the output signal, foras long as the difference pressure is below a permissible maximum valueset in advance with a certain margin, because of the follow-up set valuedelivers a signal which follows the signal which is currently switchedon. If, on the other hand, the input signal, that is DP, exceeds the setvalue, the DP control delivers a "Max DP" signal which, if the otherconditions are fulfilled, is allowed to determine the movement of theguide vanes via the selection system. The maximum permissible differencepressure in a plant designed according to the invention has been set at0.55 bar.

The status of the fuel injection concerns the operating state of thewhole PFBC plant and information about the condition (FO) must thereforebe supplied to the selection system.

If, for some reason, the gas turbine unit comes outside its permissibleworking range so that, for example, surging of the low-pressurecompressor and surging of the high-pressure compressor occur, that themaximum speed of the low-pressure compressor is exceeded, thatvibrations occur on the units, and the like, a function called GT tripis triggered. Upon a GT trip, special measures must be taken and it istherefore important that the selection system is informed of this (GT).

As mentioned above, during start-up of the plant the high-pressure unitis driven by the electric machine, connected to the common shaft and fedfrom the network. Only when the speed of the unit has reached the speedwhich corresponds to the necessary speed to be able to phase the machineinto the network, is it opened for air supply to the pressure chamberand also for gas outlet from the combustor. This opening is performedwith the intercept valve 22 according to FIG. 1. It is thereforeimportant for the selection system to know whether the intercept valvehas opened or not (IO).

Information about the states of the above functions is obtained in theform of logical 0- and 1-signals which are supplied to the selectionsystem. That part of the selection system which is to process theseinput signals must then be designed for logic processing and is for thispurpose designed such that the 0- and 1-signals have the followingmeaning:

    ______________________________________                                        Fuel injection, OFF Yes = 1, No = 0                                           Intercept valve, ON Yes = 1, No = 0                                           GT trip             Yes = 1, No = 0                                           ______________________________________                                    

The selection system 25 according to FIG. 2 can be designed in aplurality of different ways depending on the desired control andprotection strategy and whether input and output signals are analogand/or digital signals. The selection system may also be built up in amore or less integrated form within the scope of the invention. Apreferred embodiment is shown in FIG. 3. Since the input signals in thedescribed embodiment consist of both analog and digital signals, theselection system must comprise both analog and digital selectors for thelogic decisions that are to be made. All the digital selectors in FIG. 3are drawn in a position in which the activation signal from the logicinputs are 0.

Selector V1 is a first minimum selector whose output signal U1 is thesmaller of the control signal and the signal Minspeed LC. The outputsignal is passed to a maximum selector V2 whose output signal U2consists of that of the input signals thereto which is the greater.Besides U1, the input signals also comprise the signals from Min-π,Maxspeed LC and the signals U3 and U4 from a first digital selector D1,and a reference signal a1=0. When D1 is not activated, both U3 and U4will have the value zero because of the reference signal a2=0. The firstdigital selector is activated, as is clear from FIG. 3, by the signalFuel injection when this changes from 0 to 1. U3 will then be equal to aMax MW signal filtered in the filter F and U4 will be equal to the MaxDP signal.

The greatest of all the input signals to V2 will now, as the signal U2,be supplied to a second minimum selector which there are also suppliedthe signal from Max-π and a reference signal a3=100. The output signalfrom V3, that is U5 according to FIG. 3, consists of the control signalwhich is passed to the operating device of the guide vane unless thelogical signals from Intercept or GT trip are activated and requestotherwise.

If none of a second digital selector D2 and a third digital selector D3is activated, the output signal U6 from D3, because the reference a4 ofD2 corresponds to the maximum control signal to the operating device ofthe guide vane, will thus be guided towards an open guide vane ring. Insuch a state the control signal U5 to the operating device will bedisconnected. If D3 is activated, that is, if a GT trip is obtained, theoutput signal U6 from D3, because of the feedback according to thefigure, will retain the value of the signal prevailing prior to theactivation independently of the state of D2. As is clear, the signal U5also forms the follow-up set value F_(set).

The activation state of the digital selector D2 is determined by thelogical signals from Intercept and GT trip. As is clear from the figure,these signals are passed to a memory M with a subsequent time lagelement T. The relationship between the input signals to the S- andR-inputs on the memory and its output is clear from the followingsummary:

    ______________________________________                                        Intercept signal                                                                          S          0     1      0   1                                     GT trip     R          0     0      1   1                                     M signal    M1         0     1      0   0                                     ______________________________________                                    

As will be clear, it is only the combination of S=1 and R=0, that is,normal operating state with a switched-on intercept valve and no GTtrip, that may trigger D2, which can be done at the earliest, after acertain time determined by the time lag element T. This means thatduring normal operation the two control signals U5 and U6 have the samevalue, and if a GT trip should occur, D3 will be locked to the controlsignal prior to the occurrence of a GT trip.

Then when a GT trip has been corrected, that is, the GT trip signalbecomes zero, U6 is increased 100%. Only when the intercept valve isopened, does the normal control switch in U6=U5.

We claim:
 1. A method for air flow control in a PFBC plant including acombustor, a pressure vessel, a two-shaft gas turbine consisting of alow-pressure compressor LC and a low-pressure turbine LT interconnectedby a shaft as well as a high-pressure compressor HC and a high-pressureturbine HT interconnected by a shaft, wherein air is sucked into thelow-pressure compressor LC, is passed on to the high-pressure compressorHC and via an intercept valve into the pressure vessel and thecombustor, whereafter the return flow is passed via the intercept valveto the high-pressure turbine HT and further to the low-pressure turbineLT from where the return flow is released into the free environment, theinlet to LT being provided with a guide vane ring with adjustable guidevanes, said method comprising the steps of:forming a set value (G_(set))for the air flow for the current fuel based on the relationship betweenthe level of the bed material in the combustor, the bed level h, and thenecessary air flow; obtaining an actual value (G_(actual)) of the airflow based on known calculable relationships between measurableparameters; supplying the set and actual values to a regulator, andsupplying the output signal of the regulator to control an air flowcontrol means which is constituted by said guide vane ring with therotatable guide vanes.
 2. A method according to claim 1 wherein anelectric machine is connected via a gear unit to the same shaft as saidhigh pressure compressor HC and said high pressure turbine HT andwherein information about the power (MW) supplied to the machine duringstart-up of the PFBC plant is supplied to the air flow control systemtogether with information about the speed (N_(LC)) of said low pressurecompressor LC and information about the difference pressure (DP) betweenthe pressure vessel and the combustor, and wherein the air flow controlmeans includes;a) a limiting control for pressure ratio π_(HC) acrossthe high pressure compressor HC from which limiting control signals forlimiting pressure ratio (Min-π, Max-π) are delivered and switched intothe air flow control if the working point for the high-pressurecompressor HC approaches the limit area for choking or surging, b) alimiting control for the speed N_(LC) of low-pressure compressor LC fromwhich limiting control signals (Minspeed LC, Maxspeed LC) are deliveredand switched into the air flow control when the speed of low-pressurecompressor LC starts approaching the respective limit values, c) alimiting control for maximum allowed supplied power to the electricmachine during start-up, from which a limiting control signal (Max MW)is delivered and switched into the air flow control when the suppliedpower starts approaching the maximum allowed value, and d) a limitingcontrol for maximum allowed difference pressure (DP) between pressurevessel and combustor from which a limiting control signal (Max DP) isdelivered and switched into the air flow control when the differencepressure starts approaching the maximum allowed value.
 3. A methodaccording to claim 1 wherein the air flow control system is suppliedwith signals containing information as to whether fuel injection isbeing performed or not, (FO) signal, information as to whether a GT triphas occurred or not, (GT) signal, and information as to whether theintercept valve is switched on or not, (IO) signal, and wherein saidsignals FO, GT and IO are used as interlocking signals forinterlocking/blocking of the air flow control system.
 4. A methodaccording to claim 2 wherein the air flow control system is suppliedwith signals containing information as to whether fuel injection isbeing performed or not, (FO) signal, information as to whether a GT triphas occurred or not, (GT) signal, and information as to whether theintercept valve is switched on or not (IO) signal, and wherein saidsignals FO, GT and IO are used as interlocking signals forinterlocking/blocking of the air flow control system.
 5. A methodaccording to claim 4 wherein the air flow control system comprises aselection system which is supplied with the control signal from theregulator, the limiting control signals Min-π, Max-π, Minspeed LC,Maxspeed LC, Max MW and Max DP from the limiting controls and theinterlocking/blocking signals FO, GT and IO and wherein an output signalfrom said selection system corresponds to that input signal whichcurrently has the highest control priority, said output signalconstituting the control signal supplied to an operating device for theoperation of the guide vanes.
 6. A method according to claim 5 whereinregulators are provided for limiting controls and wherein all theregulators included in the control system are supplied with a follow-upset value F_(set) signal equal to the control signal supplied to theoperating device and which controls the outputs of all the regulators,except for the one that is switched on, to a value which is somewhatlower or higher than the current control signal.
 7. A method accordingto claim 6 wherein the actual value (G_(actual)) of the air flow isbased on the relationship between the speed (NH) of the high pressurecompressor HC, the temperature (T3) of the air flow before HC, thenormalized speed (N_(n)) for HC, the pressure before (P3) and after (P4)HC and the normalized compressor air flow (GN) as a function of thepressure ratio π.
 8. A device for air flow control in a PFBC plant whichincludes a combustor, a pressure vessel, a two-shaft gas turbineincluding a low-pressure compressor LC and a low-pressure turbine LTinterconnected by a shaft and a high-pressure compressor HC and ahigh-pressure turbine HT interconnected by a shaft, an intercept valve,and a guide vane ring with adjustable guide vanes being provided at theinlet to LT, said device comprising means for forming a set value and anactual value for the air flow and an air flow regulator, and wherein theguide vane ring with adjustable guide vanes is controlled by the airflow regulator and is arranged in the air flow control.
 9. A deviceaccording to claim 8, wherein for limiting control of the air flowcontrol means the device comprises a first regulator for minimum andmaximum limitation of the ratio π_(HC) between the pressure after (P₄)and before (P₃) the high pressure compressor HC and which suppliescontrol signals Minπ and Max-π, a second regulator for minimum andmaximum limitation of the speed (N_(LC)) of the low pressure compressorLC and which supplies control signals Minspeed LC and Maxspeed LC, athird regulator for maximizing the supplied power to a machine connectedto the same shaft as the high pressure compressor HC and high pressureturbine HT in connection with start-up of the PFBC plant and whichdelivers a control signal Max MW and a fourth regulator for maximizingthe difference pressure (DP) between the pressure vessel and thecombustor and which delivers a control signal Max DP.
 10. A deviceaccording to claim 8, wherein for evaluation of which control system isto be switched on or whether interlocking/blocking signals withinformation about the status of the fuel injection (FO), GT trip (GT) oran intercept valve (IO) included in the system should be determining forthe control of the guide vanes of the guide vane ring, the device isprovided with a selection system adapted for receiving input signalsfrom the control systems and the interlocking/blocking signals, theselection system comprising analog minimum selectors (V1, V3) andmaximum selectors (V2), and digital selectors (D1, D2, D3) whichdeliver, an output signal from the selection system, which is the one ofthe input signals of the selection system which has the highest controlpriority, said output signal constituting the control signal for theguide vanes of the guide vane ring.